1. Field of the Invention
This invention relates to the shaping of heat softenable sheet material, and in particular to shaping heat softened glass sheets between an upper mold and a lower, flexible ring mold.
2a. Technical Considerations
Shaped and tempered glass sheets are widely used as windows in vehicles, such as automobiles and the like. To fabricate these windows, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the window frame openings in the vehicle. It is important that the windows meet stringent optical requirements and be free of optical defects that would tend to interfere with clear viewing through the window. Any distortion in the shaping members that engage the heat softened sheet is replicated in the major surface of the sheet and may result in an optically deficient surface in the shaped sheet.
Commercial production of shaped glass sheets commonly includes heating flat glass sheets to their softening temperature, shaping the heated sheets to the desired curvature, and then cooling the shaped glass sheets in a controlled manner. In particular, the glass sheets are serially conveyed through a tunnel type furnace and heated to their heat deformation temperature and thereafter conveyed into a shaping station where the heat softened glass sheets are sandwiched between a pair of vertically aligned upper and lower shaping molds. After shaping, the molds separate with the shaped glass sheet remaining engaged against the upper mold by vacuum. A transferring tempering ring having an outline shape and conforming to the desired curvature of the glass sheets slightly inboard of its perimeter moves below the upper mold which thereafter releases the vacuum and deposits the shaped glass sheet on the ring. The tempering ring then conveys the shaped glass sheet into the cooling station for tempering.
The lower mold in such sheet shaping arrangements is generally positioned below the conveying rolls in the shaping station and has a segmented, curved sheet engaging surface complementing selected surface portions of the upper shaping mold. During shaping, the lower mold moves upward and lifts the glass sheet off the conveying rolls and into engagement with the upper mold. During engagement with the lower mold, the glass first engages the highest portions of the lower mold and then slides along the mold surface to conform therewith. As a result, there is a possibility of marking the glass as the glass slides along the lower mold face.
It would advantageous to construct a lower mold that would engage the heat softened glass sheet in a manner so as to eliminate the glass sheet sliding over the lower mold during shaping.
2b. Patents of Interest
U.S. Pat. Nos. 3,459,526 to Stickel et al., 3,476,540 to Ritter, Jr. et al., and 3,527,589 to Ritter, Jr., teach a horizontal press bending arrangement with an upper mold and a lower outline ring type mold. The lower mold is interrupted to provide clearance for the mold to move through a plane occupied by a series of spaced conveying rolls in the shaping station. The elevational configuration of the lower outline mold corresponds to a complementing portion of the upper shaping mold.
U.S. Pat. No. 4,272,274 to Frank et al., teaches a horizontal press bending arrangement with an upper vacuum mold and a slotted lower mold. The lower mold includes a plurality of smoothly curved, upwardly facing, elongated shaping members extending continuously across the entire dimension of the mold, and separated by transversely extending grooves that extend completely across the entire dimension of the shaping mold. The grooves have sufficient width and depth to permit clearance for raising the mold from below a series of conveying rolls within the shaping station to above the conveying rolls to lift and press a heat softened glass sheet against the upper mold.